In a resistance welding operation, a flexible cable formed of two or more stranded conductors is typically connected between a transformer and a welding gun having welding tips that close on opposite sides of a joint to be welded. A low voltage, high amperage electrical current (20 volts and 10,000 amps is typical) is passed through the cable and welding tips and through the joint to be welded. The high current causes fusion of the parts to be welded.
The high current through the welding cable produces a strong electromagnetic field around the cable. This magnetic field can cause a strong kick or jump in the cable. To avoid this, a cable called a "kickless cable" has been developed. This cable has a plurality of stranded conductors arranged symmetrically in a circular arrangement, with alternating conductors being positive and negative. Six conductors is a typical kickless cable arrangement. The juxtaposition of positive and negative conductors causes a cancellation in the electromagnetic fields and renders the cable "kickless".
Such cables usually are fluid cooled, with a typical cable being provided with water fittings in terminals at opposite ends of the cable, with water being pumped through one terminal and along the cable and exiting at the other terminal. The entire cable is usually covered with a waterproof jacket or sheath that encloses the conductors of the cable as well as providing a water course for cooling fluid.
In many resistance welding operations, the operation of the welding gun results in a continuous movement and flexing of the welding cable. Over time, this flexing causes the cable to work harden and also causes the fine strands of the conductors to break. This results in the cable gradually increasing in resistance over a period of time. Since the voltage in a resistance welding operation is relatively low and the current extremely high, it is essential that the resistance of the welding cable be maintained at a very low level. As the resistance increases, the amount of heat generated in the cable increases and the amount of current available for the welding tips of the welding gun reduces significantly. A drop in current eventually will produce an inferior weld.
Because of this, manufacturers usually have a predetermined time when welding cables are replaced. Some manufacturers replace a welding cable when the cable increases in resistance to 130% of its original resistance, while other companies replace cables when the resistance increases to 110% of its original resistance.
In order to determine when a cable has reached the point of replacement, the general practice is to shut down the welding line, disconnect the cable, and test the resistance of the cable. If the cable is satisfactory, the cable is reconnected and used until the next test period. If the resistance of the cable is found to be more than the predetermined percentage higher than the original cable resistance measured, the cable is replaced. The increase in cable resistance is dependent upon a number of factors, including temperature, the amount of flexing the cable has encountered, and the amount of use placed on the cable in the welding operation. Because of this, it is not possible to predict exactly when a cable is due for replacement.
An object of the present invention is to provide an improved method for testing cables and a self-diagnosing cable assembly, wherein the relative increase in resistance of a cable can be tested on the fly during cable operation, without disconnecting the cable from the welding apparatus.